Titanium dioxide is known in many different crystal forms but only three of them are commonly found in the nature—rutile, anatase and brookite.
Nanosized titanium dioxide is a material of an increasing importance in the cosmetic industry. Nano-TiO2 is perfectly inert and its special characteristics, such as absorption of UV light, make it a desirable material in UV screen lotions and other cosmetic products. It is often used in combination with nano-sized zinc oxide which also absorbs UV light.
Small particle size of TiO2 is critical for the optical transparency of the products. Particles above 70 nm in size scatter visible light causing the “white nose” effect.
While the USP grade (meeting the United States Pharmacopeia specifications), nanosized rutile has been used in the cosmetic lotions for over a decade, the use of anatase and brookite has been limited. The thermodynamically stable rutile can be manufactured with the particle size in the required nano-range, meeting the desired USP specifications at the same time. Nano-rutile is produced by several industrial processes, for example by the diluted gas phase chloride process or via titanates.
Anatase and brookite are thermodynamically unstable. Particles of anatase rapidly sinter, and the particle size distribution is quickly moving outside the nano-range at temperature higher than 600° C. There is a noticeable conversion of nano-anatase into large rutile crystals above this temperature.
Large particle size and high volume of volatiles are two major technical obstructions that disqualify the anatase crystalline materials from UV screens and other cosmetic applications. The existing USP grade anatase pigment products are heavily sintered into large aggregates of about 300 nm in size. Generally, particles in these products are by one order of magnitude larger than is the desirable size for the UV screens in cosmetics. Light scattering on these pigmentary particles eliminates the materials from UV screen applications requiring the optical transparency in the visible light. Additionally, they are often contaminated by a fraction of pigmentary rutile.
Nanosized anatase made by the low-temperature processes contains usually a significant portion of volatile impurities. It is common that materials processed at 500° C. show 5 percent weight loss when heated to 800° C. Materials prepared at 350° C. may have over 20 weight percent of volatiles, while the USP limit is only one weight percent. The USP product must be over 99 percent titanium dioxide, showing less than 1 weight percent loss on the ignition at 800° C. for 2 hours (LOI).
For these reasons, manufacturing of a pure high surface nano-anatase meeting the USP specifications in the desired quality was not possible.
An object of the present invention is to provide a manufacturing method for production of USP grade high surface area nano-anatase base. A further object is to provide the USP grade nano-anatase base suitable for sun screens and cosmetic products.